This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.
Organisms need to be able to respond to their external environment.
This is more complicated in multicellular organisms because often
intercellular communication is necessary. As the organism gets
bigger and more complex so does the system that controls response
to the external environment.
The 2 common control systems in vertebrate animals are
1. The nervous system
2. The endocrine system
The nervous sytem is often the immediate response system and controls
responses in the short term.
The endocrine system is slower acting and controls long term response.
Messages are sent in the endocrine system via cellular secretions.
Glands
Glands are groups of secretory cells.
There are 2 types of glands, exocrine and endocrine.
Exocrine glands secrete substances to the surface of the
animal. (This includes the surface of the gut and lungs.) e.g.
sweat glands and salivary glands
Endocrine glands secret substances inside the body. e.g.
testes, thyroid, and pituitary gland.
secret substances inside the body. e.g. testes, thyroid, and pituitary
gland.
There are some specialized cell secretions namely autocrine
and paracrine.
Autocrine secretions are substances that effect the the
secreting cell itself (negative feedback). Example norepinephrine
from neurons.
Paracrine secretions are substances that effect neighboring
cells. Example histamine release from mast cells.
Pheromones are yet another chemical messenger from exocrine glands that signal other animals.
Endocrine system
Endocrine glands secrete hormones that are taken up in the blood
and distributed around the body of the animal.
Are all cells effected by any one hormone?
No.
Why?
Hormones only effect cells with hormone receptors.
So how do hormones effect target cells?
The binding of a hormone to its receptor causes a biochemical change in the
cell.
Hormones can be put into 3 structural or chemical categories:
amines (e.g. epinephrine, T4)
steroids (e.g. estrogen, testosterone)
peptides (e.g. insulin, growth hormone)
Lipid soluble hormones
Steroids and thyroxine (T4) are lipid soluble and can move through cell membranes.
These hormones are insoluble in water and cannot dissolve in blood plasma.
They are carried in the blood attached to carrier proteins.
At target cells they must dissociate from the carrier protein before passing
through a cell membrane.
Steroid Hormones
Steroids are lipid soluble. Most are derivatives of cholesterol.
They move through the cell membrane and bind to a cytoplasmic receptor or receptor
bound to chromosomes.
Hormone/cytoplasmic receptor complex moves into nucleus and initiates translation
of a gene or genes.
Binding to receptor on DNA initiates translation of genes also.
Thyroxine
Thyroxine T4, moves through the cell membrane. Inside the cell it is converted
to T3.
T3 moves into the nucleus where it binds to a receptor.
T3 hormone/receptor complex forms a dimer with 9-cis-retinoic acid; they bind
to DNA and facilitate transcription of a gene.
Lipid Insoluble Hormones
Peptides, amines and prostaglandins bind to receptors in the cell membrane.
Reception leads to the activation of a second messenger or messengers that activate
other cell proteins (enzymes).
Many receptors have similar mechanisms in the cell by using the same second
messengers.
Three important second messengers are
Cyclic AMP (cAMP)
Inositol triphosphate
Calcium ions (Ca2+)
Lets look at the mechanism using Cyclic AMP (cAMP)as the second
messenger.
1. Hormone binds to receptor on membrane surface.
2. G-protein in membrane activated (GTP plus protein).
3. G-protein activates adenylate cyclase on inner surface of cell
membrane.
4. Adenylate cyclase converts ATP to cAMP in cytoplasm.
5. cAMP binds to protein kinase A, causing activation of its subunit
that activates effector protein
Effect in a cell depends upon the effector protein, which can
be different in different cells.
G-protein, adenylate cyclase and protein kinase are all enzymes.
There is an amplification process.
One G-protein molecule can activate many molecules of adenylate
cyclase.
One molecule of adenylate cyclase form many molecules of cAMP.
One molecule of protein kinase A can activate many effector proteins.
Example - Glycogen breakdown
Epinephrine (muscle) or glucagon (liver) bind to receptors.
G-protein activated and in turn activates adenylate cyclase.
Adenate cyclase converts ATP to cAMP.
cAMP activates protein kinase A.
Protein kinase A phosphorylates (activates) phosphorylase kinase.
Phosphorylase kinase activates (phosphorylates) phosphorylase
b to form phosphorylase a.
Phosphorylase a cleaves glucose from glycogen.
Note also that there is a similar mechanism for inhibiting
the cAMP second messenger system.
Another receptor when bound to an inhibitory hormone activates
a G-protein that inactivates adenylate cyclase.
Hormones and responses that use cAMP as a second messenger
Epinephrine (b receptors)
skeletal muscle - breakdown glycogen
heart - increase rate and force
smooth muscle - relax
Epinephrine ( a2 receptors)
fat cells - decrease lipid breakdown
Inositol triphosphate (IP3) and calcium are second messengers and sometimes third messengers in like cascades of enzyme activation that gretaly magnify the molecules involved after hormone reception and produce specific and often multiple cell responses.
Hormones can have more than one receptor
Epinephrine is the best example with 5, a1,
a2, b1,
b2, and b3.
b1, b2,
and b3 use cAMP as a second messenger.
a2 is coupled to an inhibitory G-protein
that decreases cAMP levels.
a1,uses inositol phosphate as the second
messenger.
Therefore hormones can have different effects on different
cells depending upon the receptors that the cell produces.
Regulation of hormone effects in any one cell can be through the
increase or decrease of receptors.
Overview of physiological effects of some hormones.
Pancreas - alpha cells produce glucagon which stimulates
breakdown of glucose in liver.
Pancreas - beta cells produce insulin which stimulates
the uptake of glucose by cells.
Thyroid gland
Produces thyroxin which effects metabolic rate, thermogenesis
and development of most cells. Promotes amphibian metamorphosis
Specialized parafollicular cells of the thyroid - produce calcitonin
which stimulates osteoblasts to build bone tissue and reduce Ca2+
in blood.
Parathyroid gland - produces parathyroid hormone that stimulates
osteoclasts to break down bone and increase blood Ca2+.
Adrenal gland
medulla produces epinephine (adrenaline) and norepinephrine
which stimulates "fight or flight response". Alters
vasoconstriction cardiac output, bronchodilation etc.
Adrenal cortexproduces testosterone and estrogens, mineralocorticoids
(regulate kidneys) and glucocorticoids (widespread effects
including increasing glucose and amino acids and anti-inflamatory.
Pituitary gland is divided into the anterior and posterior
glands.
Posterior pituitary
secretes oxytocin which effects milk let down and uterine
contraction in mammals, egg expulsion in reptiles. Antidiuretic
hormone (ADH) which increases water resorption in kidneys.
These hormones actually produced by nerve cells in the hypothalmus
and are transported down axons.
Anterior pituitary
produces many hormones that control hormone release in other
organs such as thyroid stimulating hormone (TSH), follicle
stimulating hormone (FSH), luteinizing hormone (LH) (gonadotropins),
adrenocorticotropin, melanocyte stimulating hormone, and growth
hormone (somatotropin) and prolactin (mammary gland development).
Hormones from the anterior pituitary are in turn regulated by
hormone releasing or hormone inhibiting hormones produced
by neurons in the hypothalmus.
e.g. gonadotropin releasing hormone (GnRH), growth hormone releasing
hormone (GHRH), growth hormone inhibiting hormone (GHIH)(somatostatin)
etc.
The hypothalamus provides the integration point between our nervous system and our endocrine system.
Can your thoughts effect your health?
Stress response
Stress causes release of CRH - the fight or flight response.
CRH stimulates sympathetic nerve response, and adrenal cortex (glucocorticoids,
cortisol)
Widespread effects:
inhibit insulin
muscle/bone degeneration
protein, fat catabolism
inhibit digestion
inhibit TSH, GH, FSH, LH
This page was created by Peter King. Please contact the author
at pking@fmarion.edu with
comments.
http://people.fmarion.edu/pking/vertphys/endocrine.html
Last edit January 10, 2011.
Copyright Peter King.